The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Accurate downlink power control is highly important in a system that utilizes multiple satellites transmitting different information at the same frequency to one user. The user receiver that decodes information from one satellite at a time, when subjected to an ensemble of many signals at the same frequency, ultimately interprets the rest of all signals as background noise, thus reducing the energy-per-bit to noise ratio and increasing the probability of error. Also, maintaining accurate drive levels over a range of operating temperatures into a non-linear radio frequency (RF) power amplifier of a satellite transponder is highly desirable as it eliminates the phase (delay) uncertainty caused by the amplitude modulated/phase modulated (AM/PM) conversion factor of its non-linear devices.
It will also be appreciated that user range error (URE) is one highly important performance parameter of the GPS system. Maintaining accurate drive levels over temperature into a non-linear RF power amplifier of a satellite transponder eliminates the phase (delay) uncertainty caused by the AM/PM conversion factor of its non-linear devices. This means enhanced position fix accuracy for both commercial and military users.
Existing solutions for power measurements of composite QPSK (quadrature phase shift keyed) signals have been developed for receivers where the signals are not known in their entirety. These designs are generally highly complex since the QPSK signal has to be separated into its quadrature components, the codes for all constituents re-generated in the receiver, and finally the separated signals need to be multiplied by the proper codes before being filtered and submitted for sampling and power measurement. After typically thousands of samples, a mean power is obtained with its associated deviation or uncertainty.
For guaranteeing accurate power downlink power control, one specific previously used technique has involved monitoring at the ground stations the power settings of all components of the composite QPSK signal. More specifically, this involves demodulating and sampling the composite QPSK signal, recalculating the inter-related power settings of the components of the QPSK signal and uploading the new power settings. This technique requires more than one cycle due to the non-linear effects of the RF power amplifier and has a minimum inherent error of approximately 0.5 dB. Since the code power measurements have to be made from a distance of approximately 26,000 km and the atmosphere is an unstable propagation media due to its different layers at different temperatures and varying densities, the accuracy in the code power setting is generally quite limited (typically worse than 0.5 dB).
For the problem of minimizing user range error (URE), there presently is no satisfactory solution. The satellite transponder and its constituent subsystems will typically vary in temperature, which presents to the RF power amplifier small power fluctuations at its input. These small power fluctuations typically result ultimately in phase (i.e., delay) changes of the transmitted signal. This is because electromagnetic energy travels at the speed of approximately 30 cm per nanosecond. Thus, one nanosecond of delay uncertainty translates into a roughly 30 cm position fix error to the user receiver.